Django: Advanced Models

Chapter 10Advanced Model

Anne LaiDjango Girls meetup

2015/10/8

Django Book

• Related Objects

• Making Changes to a Database Schema

• Manager

• Model Methods

• Executing Raw SQL Queries

Outline

Recall of Chapter 5

• Define models • Use the database API to create,

retrieve, update and delete records

Related Objects

Related Objects

• ForeignKey

• ManyToManyField

Related Objects

Related Objects

✐return QuerySet values instead of model instances

Related Objects

✐ book_set = lowercase(Book) + “_set”QuerySet

Making Changes to a Database Schema


• syncdb

- sync model to the database

- merely creates tables that don’t yet exist in database

- does not sync changes in models or perform deletions of models


• Adding Fields

• Removing (normal/ Many-to-Many) Fields

• Removing Models

Adding Fields

Development Environment

1. Add the field to your model

✐a database column will contain

NULL values when first created.

Adding Fields

2. Run manage.py sqlall [app] to see the new CREATE TABLE statement for the

model


✐column definition for the new field

Adding Fields

3. Start database’s interactive shell

(e.g., psql or mysql, or you can use manage.py dbshell)

Execute an ALTER TABLE statement that adds your new column


NULL column

NOT NULL column

Adding Fields

4. Verify with Python interactive shell manage.py shell


✐ If a model contains a field that has not yet

been created in the database table, it will cause

an error happens at code execution time.

Adding Fields


1. Add the field to your model.

2. Run manage.py sqlall [app] to see the new CREATE TABLE statement for the

model. Note the column definition for the new field.

3. Start database’s interactive shell (e.g., psql or mysql, or you can use

manage.py dbshell). Execute an ALTER TABLE statement that adds your new

column.

4. Verify with Python interactive shell manage.py shell by importing the

model and selecting from the table (e.g., MyModel.objects.all()[:5]).

Adding Fields

Production Server

1. Start database’s interactive shell (e.g., psql or mysql, or you can use

manage.py dbshell)

2. Execute the ALTER TABLE statement

3. Add the field to your model. If you’re using source-code revision control

and you checked in your change in development environment step 1,

now is the time to update the code (e.g., svn update, with Subversion) on

the production server

4. Restart the Web server

Removing Fields

1. Remove the field from model and restart the Web server.

2. Remove the column from your database, using a command like this:

✐ Order sensitive

1. Remove the ManyToManyField from model and restart the Web server.

2. Remove the many-to-many table from your database,

using a command like this:

Normal Field

Many-to-Many Field

Removing Models

1. Remove the model from your models.py file and restart the Web server.

2. Remove the table from your database, using a command like this:

✐ Order sensitive

Manager

Manager

• Adding Extra Manager Methods

• Modifying Initial Manager QuerySets

• Multiple Managers

✐ Managers = database queries

Adding Extra Manager Methods

Adding Extra Manager Methods

✐ manager itself (objects)

✐ extends django.db.models.Manager

✐ replacing the “default” manager

Modifying Initial Manager QuerySets

QuerySet

Multiple Managers

✐ Django interprets the first Manager defined in a class as the “default” Manager

Model Methods

Model Methods

http://www.python.org/download/releases/2.2/descrintro/#property

✐property(fget=None, fset=None, fdel=None, doc=None)

• Model Method:

- “row-level” functionality to objects

- Act on a particular model instance

• Managers:

- “table-wide” or “table-level”

Comparison

Executing Raw SQL Queries

Executing Raw SQL Queries ✐ current db connection

✐use placeholders rather than adding parameters directly within the SQL

standard Python “DB-API”: https://www.python.org/dev/peps/pep-0249/

Thank you

Django: Advanced Models

Technology

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